1. Field of the Invention
Aqueous dispersions of polymeric compounds and their use in formulating water borne ambient cure coatings are described herein.
2. Background of the Related Art
Water-based polymeric systems are becoming increasingly important, especially in coating, adhesive, and ink applications due to environmental laws on restricted use of volatile organic compounds.
One type of coating widely used in the area of OEM (Original Equipment Manufacture) and refinish coatings are the two-component coatings based on acrylic polyols and polyisocyanates. These coatings are employed for ambient-cure or low-bake applications over metal, plastics, wood and a variety of other types of substrates. The conventional types of coatings based on acrylic polyols and polyisocyanates contain relatively large amounts of volatile organic compounds (VOCs) used as solvents.
High-solids coatings based on acrylic polyols and polyisocyanates have been developed in order to reduce the VOC levels of these types of coatings. The higher-solids coatings are desirable for their lower VOC content but have been found to suffer from a number of drawbacks such as difficulty in achieving balance of cure-speed and pot-life, good wetting of all types of substrates, inability to match the performance of conventional solvent-based coatings. Typical --NCO/--OH levels are greater than 1.5, which results in environmental and film property problems. Incorporation of ionic or hydrophilic sites on the polyurethane, use of surfactants in the acrylic polyol emulsion and dispersible polyisocyanate, and addition of other additives result in an increase in the water sensitivity of the coatings. Generation of microfoaming caused by the dispersion process, reaction of the isocyanate groups with water, or spray coating application can lead to a reduction in clarity, gloss, distinction of image (DOI), and pinholing in the final film. Another disadvantage with prior systems is that special high shear mixing equipment is commonly needed with aqueous two-component systems to prevent gel formation caused by incompatibility and viscosity differences of the acrylic emulsion and the dispersible polyisocyanate. Other disadvantages associated with waterborne two-component polyurethane systems are poor re-wetting characteristics, different rheology than solvent-borne two-component systems, pH sensitivity, and catalyst compatibility. Accordingly, there is a need to develop alternative low-VOC coatings based on acrylic polyols and polyisocyanates that do not exhibit the aforementioned disadvantages.